The results of members defined with a TRUSS specification is not correctly accounting for the effects of transverse loads when applied as member loads.
In order to prevent a member from transferring any moment across the start or end of a member, it is possible to specify releases using a MEMBER RELEASE command in each of the three local axes. However, STAAD.Pro additionally includes a quick method for defining a member as having releases at both ends simultaneously, by assigning a TRUSS specification.
In the update 5 release of STAAD.Pro CE version, a modification was introduced to assist with the use of self-weight on members with a TRUSS specification as this loading equates to a UDL on the member. This modification however did not fully account for the axial forces that may be present in such members.
The current recommendation is not use TRUSS member specification on members that have transverse loads and are intended to carry axial forces alone. Instead it is strongly advised to use MEMBER REEASE commands in order to account for the releases in the 3 rotational degrees of freedom at the start and ends of such members.